Technical Field
The present invention concerns a method an apparatus for distillation of a liquid near or above the critical point of the liquid, particularly for use in desalination of salt water or purification of organically tainted waste water from industry and agriculture.
Liquid such as saline drinking water can be purified of solid substances using distillation by retaining the non-volatile substances of the liquid as a residue in the distillation container and allowing the vapor, which evaporates from the liquid, to pass into the condensation part, in which it is condensed as a distillate possibly containing volatile substances and the components of the air dissolved therein.
Evaporation of the vapor from the liquid comprises a phase shift from liquid to vapor, just as the condensation of the vapor from vapor to liquid comprises a phase shift from vapor to liquid, both of which being driven by forces which are caused by differences in pressure, temperature or concentration above a liquid-vapor interface. At pressures and temperatures below the critical point of the liquid, the liquid may be brought from liquid phase to vapor phase at a constant temperature by addition of the necessary evaporation heat. However, at pressures and temperatures above the critical point this phase shift can take place without addition of heat, because the evaporation heat at these super critical pressure and temperature conditions is zero. As a case in point it may be mentioned that the evaporation heat is 2257 kJ for 1 kg of water at 1 atm and 100.degree. C., whereas it is 0 kJ at the critical point at 221,20 bars and 374.15.degree. C. "Distillation below the Critical Point"
To obtain optimum energy utilization in distillation of a liquid below the critical point, it is known to heat the liquid using the energy, including the evaporation heat, available in the vapor.
It is known from e.g. "Principles of Distillation", Part A and B, Editors K. S. Spiegler and A. D. K. Laird, Academic Press Inc., 1980, New York to perform distillation by thermal single--or multi-step evaporation processes, in which the energy available in the discharge currents of distillate and residue contributes to heating the feed stream. For such processes the driving force is provided for the heat transfer by a temperature difference between the discharge streams and the feed stream, which is substantially established by lowering the vapor pressure of the liquid.
It is moreover known, ibid, to perform distillation of a liquid by evaporation processes in which a temperature difference between the discharge streams and the feed stream is established by condensing and cooling the vapor from a compressed state, thereby creating a higher vapor pressure for the liquid.
A common drawback of these evaporation processes is that they all require addition of evaporation heat, and that reuse of the added energy requires difficult and cumbersome multi-step processes or vapor compression processes, the latter requiring compressors which, frequently with low efficiencies, are to be capable of compressing large volumes of vapor at relatively low pressures.
Another drawback is that it is not certain that the distillate is sterile.
Accordingly, there is a need for a liquid distillation technique wherein a lower energy consumption is achieved than by the heretofore known methods, while ensuring that the distillate is sterile.